CIS 260 TestClass.java

Code

/*
    TestClass.java
    David Klick
    2014-02-09
    CIS 260
    
    This program demonstrates some basic access modifier and class concepts.
*/

public class TestClass {  // automatically extends Object
    static int x;    // only one x, shared by all objects of TestClass

    int y;           // each object of TestClass gets its own y variable
    public int a;    // a is accessible to any other class
    private int b;   // b is accessible only to TestClass objects
    protected int c; // c is accessible to TestClass objects, any
                     //    subclass objects, and any classes in the
                     //    same package
    final static double PI = 3.2;  // innacurate constant declaration
   
    // of the above variables, only x will be automatically initialized (to 0)
    private void incXY() {
        // since this is an instance method, it has access to both
        //   instance and static variables
        x++;
        y++;
    }

    public static void main(String[] args) {
        // this method only has access to the static members
        x = 7;   // works
                 // but "a = 17;" wouldn't work
      
        int y = 18; // we can set y to 18, but it is not the same y that
                    //   is the instance variable declared previously -
                    //   we just created a new one

        // we can't call incXY() directly since it is an instance
        //    method (part of an individual object) and we are in a static
        //    method which has no direct reference to an object
        // we can however create an object and use it to call incXY()
      
        TestClass t = new TestClass();  // creates a new one of us
        t.incXY();    // this works
        System.out.println("x=" + x + ", y=" + y + ", t.y =" + t.y);
    }
}

/*
    Output from sample run:
   
    x=8, y=18, t.y =1
*/